Globally, Fusarium graminearum is an important plant pathogen, attacking a wide range of plant species including many important crop plants such as corn (ear and stalk rot), barley, and wheat (head blight). Favourable environmental conditions (conducive temperatures and high humidity) can result in Fusarium epidemics and millions of dollars lost in crop revenues. Fusarium graminearum infection in the cereals reduces both the yield and quality of the grain. The reduction of quality is a result of the mycotoxins produced by this species of fungus; these fungal toxins remain in the contaminated cereal after harvest and pose serious health risks to animals and humans who may consume the grain.
Low levels of contamination in non-epidemic years still account for 5% grain losses to Ontario corn farmers, a figure which translates into approximately $27 Million to the swine industry which uses this corn for feed. In epidemic years, this dollar figure can double or triple. These direct losses to growers include the crop and animal losses associated with reduced feed and poorer quality feed. Overall, the FOA of the United Nations estimates that 25% of the world's food crops are affected by mycotoxins each year (Mannon and Johnson, 1985, Fungi down on the Farm, New Scientist 105: 12-16). Fusarium mycotoxins are found in all the major cereal species including corn, wheat, barley, oats, rye and others. The disease is most prevalent in temperate climates.
Mycotoxins, or fungal toxins, are produced by many species of fungi. The species Fusarium graminearum is capable of producing a class of compounds known as the trichothecenes. This large family of sesquiterpene epoxides are closely related and vary by the position and number of hydroxylations and substitutions of a basic chemical structure. The major trichothecene produced by Fusarium graminearum is deoxynivalenol (DON) also known as vomitoxin for its ability to induce vomiting. These chemicals are potent eukaryotic protein synthesis inhibitors, toxic to both humans and animals, and other organisms such as plants.
Due to their toxicity, safety threshold values have been recommended for DON mycotoxin contamination in grain used for human and animal feed. These values are currently established at 2 ppm of DON in non-staple food stuffs for human consumption, 0.1 ppm in breadwheat and staple foodstuffs, and 1 ppm in infant foodproducts. For livestock feed, the recommended levels are 1 ppm of the complete diet for swine and lactating cattle and 5 ppm for poultry and non-lactating cattle (Underhill, CFIA Fact Sheet, Mycotoxins, 1996). The danger to livestock producers is that if livestock animals are fed contaminated grain they suffer severe health hazards, which include reduction of feed intake, reduced growth rate, reduced fertility, immunosuppression, diarrhea, vomiting and possible death. Some of these effects are directly observable and therefore measurable, such as weight loss, whereas other effects, such as immunosuppression, are more subtle and less quantifiable. In general, a reduction of 10 to 20% of the farrowing rate of swine combined with a 10 to 20% reduction in animal growth rates can cause an approximate 17 to 44% reduction in profit margin for hog producers. The effects of mycotoxins on poultry and cattle are less quantified since both of these species are less sensitive to DON contamination in their feed, and detailed economic threshold assessments have not been made.
During years of Fusarium epidemics, grain which is above the safety threshold of 2 ppm DON for human consumption must be downgraded to animal feed. If the grain contains more than 10 ppm DON, it is rendered unfit for animal feed and must be disposed of. Since many farmers use their own cereals for on-farm animal feed, and they may not be capable of assessing the level of mycotoxin contamination of the grain, a considerable amount of DON-contaminated feed is used. Thus it is important to minimize the level of trichothecenes in food stuffs, which can be accomplished by controlling the outbreaks of Fusarium species in cultivated cereal species.
Chemical treatment has been used in the past to control trichothecene biosynthesis. One such inhibitor is ancymidol, which has been described in U.S. Pat. No. 4,816,406. However, in the present environment, it is desirable to avoid chemical control, especially in food stuffs. Thus, there is a need for a method of controlling the outbreaks of Fusarium species, particularly F. graminearum by using non-chemical methods.
Trichothecenes have been shown to act as virulence factors in wheat head scab. This was demonstrated by inoculating wheat heads with trichothecene-nonproducing mutants of F. graminearum in which the first gene specific to the trichothecene biosynthetic pathway had been disrupted through genetic engineering (Desjardins et al., 1996, Mol. Plant-Micr. Int. 9:775-781). In two years of field trials, the trichothecene-nonproducing strains were less virulent than the trichothecene-producing progenitor or revertant strains, as measured by several disease parameters. Similar results have been obtained from the inoculation of field-grown corn with these trichothecene-producing and -nonproducing Fusarium strains. Therefore, increasing the tolerance of wheat or corn to the effects of trichothecenes should lead to reduced disease.